Chemistry and electrochemistry of steam generator tube/support plate crevices

Corrosion that occurs in the tube/support plate crevices of Pressurized Water Reactor (PWR) Steam Generators has proven to be an important operating problem. Among other factors, localized corrosion occurs due to the segregation and concentration of impurities in the tube/support plate (T/SP) and tube/tube sheet (T/TS) crevices. Although the steam generator feedwater contains impurities at extremely low levels, impurities accumulate (along with corrosion products) in cracks, crevices and sludge piles by a thermohydraulic (or concentration) mechanism. In the case of T/SP crevices, the temperature gradient across the SG tube produces boiling in the crevice, which in turn results in the flow of water into the cavity to replace exiting steam. The coupling of diffusive and convective fluxes within the crevice, and the lower solubility of the impurities in steam compared with water, leads to impurity concentrations in the crevice that may be orders of magnitude greater that those in feedwater. In this work, a physico-electrochemical model has been developed to describe the evolution of the physico-hydrodynamic, chemical and electrochemical properties of boiling T/SP crevices in PWR steam generators. The Coupled Environment Crevice Model (CECM) incorporates thermal hydraulic, mass transport and electrochemical effects. We have developed also a T/SP crevice simulator, which is capable of simulating the thermal hydraulic and galvanic processes that occurs in SG crevices in service. Measured coupling currents flowing between various components (Alloy 600 tube/carbon steel support plate within the crevice, Alloy 600 tube external to the crevice/Alloy 600 tube internal to the crevice, etc.) are found to be in good agreement with theory with respect to the direction of flow.